230 Dr. Alex. Hodgkinson on 



material and independent of such amount. It is to this 

 line I have applied the term "primitive absorption line," 

 because it is the first to appear in a medium of gradually- 

 increasing thickness when light is passed through it. How 

 then is such primitive line to be determined ? 



If we take a certain thickness of any absorbing body, 

 say blood, and if such film is very thick, we get, when 

 examined with the spectroscope, such a spectrum as is 

 represented by Figure i, Plate I. ; the whole of the light 

 is absorbed, excepting the red rays below D. As the film 

 is made thinner, a broad absorption band is distinguished 

 extending from a little above D to E, and, by reducing the 

 thickness still more, this broad band is resolved into the 

 two characteristic absorption bands of oxygenated or arterial 

 blood. If we trace either of these bands down through 

 successively thinner films, we find they become narrower 

 and fainter until they become invisible, and, therefore, in 

 spectrum (7) we have no absorption band depicted. This 

 fading away of the band occurs long before the edges of the 

 same band join each other, or the band becomes a mere line. 



If, now, I take a hollow prism, carefully made of very 

 thin glass, and, having filled it with a solution of blood, 

 place it before the slit of the spectroscope with the re- 

 fracting angle of the hollow prism at right angles to the 

 long axis of the slit, when looked at through the slit only, 

 the prisms of the instrument being temporally removed to 

 allow of this, it presents the appearance seen at (1) Plate II. 

 At the base of the hollow prism, where the light has to pass 

 through a great amount of absorbing fluid, the slit appears 

 of a dark red, whilst, as we approach the apex of the prism 

 the slit appears lighter and lighter, until at the apex, which 

 is marked on the diagram as the " central line," it appears 

 almost colourless. If, now, the light from this slit is 

 allowed to pass through the prism of the spectroscope, we 

 observe the appearance depicted in the adjacent Figure 2. 



